1. Field of the Invention
The invention relates to techniques for the attenuation of noise and/or the suppression of electromagnetic interference (EMI) in electronic equipment that includes, but is not limited to, personal computers (PCs). More particularly, the invention relates to an embellishment to a circuit arrangement that incorporates a discrete bypass capacitor and series-resonant impedance for noise attenuation and/or EMI suppression at frequencies above, for example, 100 MHz.
2. Description of the Related Art
Computer systems in general, and personal computer (PC) systems in particular, have attained widespread use in providing computer power to many segments of modem society. A conventional PC system can usually be defined as a desktop, floor standing, or portable microcomputer that includes a system unit having a system processor and associated volatile and non-volatile memory, display monitor, a keyboard, one or more diskette drives, a fixed disk storage device and an optional printer.
PC systems may be considered information handling systems that are designed primarily to provide independent computing power either to a single user or to a relatively small group of users, as in the case of personal computers that serve as computer server systems. Accordingly, such systems are intended to be inexpensively priced for purchase by individuals or small businesses. A PC system may also include one of a plurality of peripheral or I/O devices that are coupled to the system processor and that perform specialized functions. Examples of I/O devices include modems, sound and video devices or specialized communication devices. Mass storage devices, such as hard disks, CD-ROM drives and magneto-optical drives, are also considered to be peripheral devices. Computers producing multi-media effects, i.e., sound coupled with visual images, are in increased demand as computers become used for artistic endeavors, for entertainment, and for education. In addition, the use of sound makes game playing more realistic and helps reinforce knowledge and make educational programs more enjoyable to use. Digital effects and music can also be created on the computer and played through attached speakers without the need for additional musical instruments or components.
A significant consideration in the design and fabrication of compact (and therefore densely assembled) PCs and other high-speed digital equipment is the need to minimize the effects of ringing, crosstalk, radiated noise and other forms of electromagnetic interference (EMI). However, design approaches seeking to minimize EMI effects are generally unsusceptible to straightforward circuit analysis. In fact, although entire textbooks have been devoted to techniques for combating EMI, the subject continues to be viewed as “black magic”. See, for example, Howard Graham, High-Speed Digital Design, Prentice Hall PTR (1993).
High-speed digital circuits and systems frequently draw large transient currents during short intervals, when, for example, logic circuits and devices change state. Often logic transitions take place with brief rise and fall times, under the control of increasingly high-frequency clock signals. Because realizable voltage sources for digital circuitry are characterized by series resistances and inductances, bypass and decoupling capacitors are commonly relied on to supply transient current requirements during transition intervals. The coupling capacitors are typically electrically connected between a voltage supply and ground and serve to mitigate the effects of the nonzero voltage supply source impedance. The decoupling capacitors, therefore, tend to maintain the output of the voltage supply by providing a significant portion of the transient current.
However, the ability of commercially available capacitors to supply current at high frequencies is limited by the parasitic lead inductance that is characteristic of such capacitors. In addition to the inductance associated with capacitor leads, the finite inductance of each via that may be used, for example, to attach a power supply plane to a ground plane introduces a small, but measurable inductance.
Accordingly, U.S. patent Ser. No. 09/491,290, Digital Circuit Decoupling for EMI Reduction, filed Jan. 25, 2000, by Jeffery C. Hailey and Todd W. Steigerwald, assigned to the same assignee as is this Application (hereby incorporated in its entirely for all purposes), addresses EMI that is related to bypass capacitors themselves, and particularly addresses EMI that results from the inductance that is inherent is commercially available, discrete bypass and decoupling capacitors. That Patent Application discloses a printed circuit board assembly in which at least two decoupling capacitors are used to decouple (to ground) transient currents that result from, for example, logic transitions in high-speed digital circuitry. The decoupling capacitors are physically arranged, and electronically connected between a power plane and a ground plane, so that transient currents flow in respectively opposite directions through the capacitors, thereby maximizing the capacitors' mutual inductance, and thus minimizing the EMI generated by the capacitors.
It may be properly inferred from the above, that it is widely appreciated in the design a manufacturer of compact and densely populated electronic equipment, such as personal computers, that attenuation of spurious noise signals and EMI is a significant system design consideration. As personal computers become more compactly packaged, and as the data and/or clock rates at which PCs operate continue to escalate, the generation of and susceptibility to high-frequency signals (e.g., signals at 100 MHz and above) similarly exacerbate. A certain degree of noise rejection and EMI suppression is conventionally achieved by judicious placement of discrete bypass capacitors between points on significant signal or voltage paths and ground. To the extent that the impedance of the bypass capacitor tends to approximate zero at high frequencies of interest, the noise or EMI at these frequencies can be suppressed or attenuated by coupling these signals through the bypass capacitor to ground.
However, the electrical characteristics of commercially available bypass capacitors that may be obtained at pragmatic prices deviate markedly from those of an “ideal” capacitor. Accordingly, because of equivalent series inductances and resistances, the magnitude of the impedance of standard capacitors may actually increase with frequency. Nevertheless, the non-ideal nature of such capacitors may be exploited. That is, a commercially available ceramic chip capacitor having a nominal value of 0.1 μf provides effective bypass at 10 MHz, but may also exhibit series resonance at, say, 100 MHz or above. As a result, a particular chip capacitor part may be empirically selected to provide effective bypass at one range frequencies and, because of stray capacitance and/or inductance, to present resonance at a frequency of interest, which may be, for example, the clock rate of the PC, or a harmonic thereof. Unfortunately, as the clock frequencies and data rates that are encountered in PC design rise, discrete capacitors that exhibit a “parasitic” series resonance at frequencies of interest have not appeared to be readily available.
Accordingly, what is desired is a circuit technique that may be used in conjunction with commercially available discrete bypass capacitors to provide suppression and attenuation of noise and EMI a frequencies above about 100 MHz. The technique must be cost-effective and compatible with existing constraints applicable to printed circuit board arrangement and PC packaging.